[sqlite] Implement RecoverPager in terms of sqlite3_file.

SQLite's Pager structure is for internal use only, whereas sqlite3_file
is a documented interface available to SQLite clients or extensions.
Where Pager accesses the page cache with fallback to the disk file,
sqlite3_file only accesses the disk file.  This should not have
significant performance impact (recover.c only reads a given block
once), but the new version may not see modified pages in the page cache.
For this reason, a shared lock is taken to guarantee that the on-disk
format matches anything in the cache.

BUG=584407

Review URL: https://codereview.chromium.org/1677193003

Cr-Commit-Position: refs/heads/master@{#374820}